Limiting Factors

¨A limiting factor is a factor that causes
population growth to decrease.

¨Some limiting factors that can affect a population are:

ØCompetition

ØPredation

ØParasitism and disease

ØDrought and other extreme climates

ØHuman disturbances

¨A resource base that is limited can also affect the long-term survival
of a species.

vThe panda population is one of the most endangered species today
because pandas depend on bamboo for food and the forests are being cleared for timber and farmland.

Density-Dependent Factors

¨A density-dependent limiting factor is a limiting
factor that depends on population size.

¨These factors operate most strongly when a population is large and
dense.

¨They do not affect small, scattered populations as greatly.

¨Density-dependent limiting factors include:

ØCompetition

ØPredation

ØParasitism and disease

Competition

¨When populations become crowded, organisms compete with one another
for food, water, space, sunlight, and other essentials.

¨Competition among members of the same species is a density-dependent
limiting factor.

¨The more individuals living in an area, the sooner they use up the
available resources.

¨Competition can also occur between members of different species.

¨This type of competition is a major force behind evolutionary change.

¨The species may evolve to occupy separate niches.

¨No two species can occupy the same niche in the same place at the
same time.

Predation

¨A predator-prey relationship is the regulation
of a population by predation.

vThe interaction between wolves and moose on IsleRoyal is a classic example.An
increase in the moose population-the prey-is quickly followed by an increase in the wolf population-the predators.As the wolves prey on the moose, the moose population falls. Then a decline in the wolf population because
there is less for the wolves to feed upon.So, the moose have fewer enemies and
that population rises again.

Parasitism and Disease

¨Like predators, parasites take nourishment at the expense of their
hosts, often weakening them and causing disease or death.

¨So, parasites can also limit the growth of a population.

Density-Independent
Factors

¨Density-independent limiting factors affect all populations in similar ways, regardless of the population size.

¨Density-independent limiting factors include:

ØUnusual weather (extreme hot or cold)

ØNatural disasters (storms or hurricanes)

ØSeason cycles (droughts)

ØHuman activities – damming rivers, clear-cutting forests

¨Many species show a characteristic crash in population size.

¨After the crash, the population may soon build up again, or it may
stay low for some time.

¨Environments are always changing, and most populations can adapt
to a certain amount of change.